A vehicle, such as an automobile, is provided with systems, including a driving/braking system that transmits a driving force from a propulsive force generating source, such as an engine, to a wheel or imparts a braking force, a steering system (steering control system) for controlling steering control wheels of a vehicle, and a suspension system that elastically supports a vehicle body on wheels, as main mechanisms. Furthermore, in recent years, there has been known a vehicle provided with a variety of electric or hydraulic actuators to actively (positively) control the operations of the actuators according to a traveling condition or an environmental condition or the like rather than only passively operating the aforesaid systems in response to operations (man-caused operations) of a steering wheel (driver's wheel), an accelerator (gas) pedal, a brake pedal or the like performed by a driver (refer to, for example, 6.8.1 of page 220 of “Automotive Engineering Handbook—Basics and Theory (Vol. 1)/The Society of Automotive Engineers of Japan (published on Jun. 15, 1992)” (hereinafter referred to as non-patent document 1)).
Regarding, for example, a steering system, a control method called a model following method is shown in FIGS. 6-99(a) on page 225 of the aforesaid non-patent document 1. According to this control method, a steering control angle of a steering wheel operated by a driver is input to a reference model in which the steering control response characteristics of yaw angular velocities and lateral accelerations have been set in advance. Then, an input of a vehicle model is determined so as to make the vehicle model follow an output of the reference model, and the determined input is additionally input to an actual vehicle (an actual steering system), thereby operating the actual vehicle steering.
However, the technology shown in the aforesaid non-patent document 1 has been suffering from the following inconveniences. The behaviors of an actual vehicle are subjected to a variety of disturbance factors, including changes in a friction coefficient of a road surface. Meanwhile, it is virtually difficult to construct a vehicle model expressing behaviors of the vehicle by taking every disturbance factor into account, and there is no other choice but to construct it by assuming a certain standard environmental condition. Furthermore, even if a vehicle model is constructed by taking a variety of disturbance factors into account, various parameters defining a behavior of the vehicle model will develop errors (modeling errors). Therefore, according to the technology shown in FIGS. 6-99(a) on page 225 in non-patent document 1, a variety of disturbance factors acting on an actual vehicle or changes therein cause a significant difference (mismatch) between a behavior of the actual vehicle and a behavior of a vehicle model to take place in some cases. In such a case, a control input (manipulated variable) that does not match a behavior of the actual vehicle will control an operation of an actuator of the vehicle or the operation of the actuator will be restricted by a limiter, making it difficult to ideally control the operation of the actuator.
In a conventional vehicle wherein a driving/braking system, a steering system or a suspension system is actively controlled through the intermediary of actuators, generally, state amounts (a vehicle speed, an engine speed, and the like) of the vehicle and a vehicle traveling environment (a friction coefficient of a road surface and the like) are detected through sensors or estimated by observers or the like, and on the basis of the detection values and estimated values, the manipulated variables of the actuators (control inputs to the actuators) are determined using a map or the like that has been set beforehand on the basis of traveling experiments or the like. With such a technology, however, it frequently happens that a detection error or an estimation error in a state amount or a traveling environment of a vehicle causes a control input (manipulated variable) that does not suit a behavior of the actual vehicle to control an operation of an actuator or to restrict an operation of the actuator by a limiter. This makes it difficult to ideally control the operations of the actuators.
The present invention has been made in view of the background described above, and it is an object thereof to provide a control device for a vehicle that is capable of enhancing robustness against disturbance factors or changes therein while carrying out control of the operations of actuators that suit behaviors of an actual vehicle as much as possible.